The theodolite requires two types of adjustments to ensure accuracy during measurements:
- Temporary Adjustments
- Permanent Adjustments
1. Temporary Adjustments
Temporary adjustments are the essential steps that must be performed at each instrument station prior to taking any measurements. These adjustments ensure that the theodolite is positioned correctly and ready for precise observation.
The key steps involved in the temporary adjustments of a theodolite are:
(i) Setting up and Centering the Theodolite
(ii) Levelling the Theodolite
(iii) Elimination of Parallax
(i) Setting up and Centering the Theodolite
The process of setting up a theodolite involves both centering the instrument over a ground mark and performing an initial, approximate leveling using the tripod legs. These steps are crucial to ensure the accuracy of the instrument’s measurements.
Centering
Centering refers to the alignment of the theodolite’s vertical axis, which is represented by a plumb line, so that it passes precisely through the station mark on the ground. This operation is performed as follows:
(i) Attach a plumb bob to the hook at the bottom of the theodolite to establish the vertical axis.
(ii) Position the theodolite over the ground station mark by spreading the tripod legs, ensuring the telescope is at a convenient working height.
(iii) Adjust the tripod legs, both radially and circumferentially, until the plumb bob hangs within approximately 1 cm of the station mark.
For fine-tuning, use the center-shifting mechanism (if available) to achieve precise centering.
Approximate Leveling Using the Tripod
Before completing the centering process, it is essential to ensure that the tripod head is approximately level. Failure to do so can cause the centering to be disturbed when leveling adjustments are made later. The leveling can be performed by:
- Referring to the small circular bubble located on the tribrach (if available).
- Judging the level by eye to ensure the tripod head is as level as possible before final leveling.
This initial leveling helps prevent any misalignment when further leveling adjustments are made after centering.
(ii) Levelling the Theodolite
Leveling a theodolite involves adjusting the instrument so that its vertical axis is perfectly vertical. Once approximate levelling and accurate centering have been achieved, precise levelling is carried out using the plate levels. The method for levelling a theodolite depends on the number of foot screws on the instrument. Below is a description of levelling with a three-screw head.
(a) Levelling with a Three-Screw Head
The following steps outline the process of levelling a theodolite with a three-screw base:
Positioning the Plate Level
Rotate the horizontal plate so that the longitudinal axis of the plate level is parallel to an imaginary line joining any two of the levelling screws.Adjusting the Bubble Using Two Screws
Simultaneously turn the two foot screws in opposite directions—either inward or outward—until the bubble of the plate level is centered. The movement of your left thumb will indicate the direction in which the bubble will move.Rotate the Instrument by 180°
Turn the theodolite 180° in azimuth to observe the position of the bubble.Correcting the Bubble Position
If the bubble shifts to a new position after the rotation, adjust the same two foot screws to move the bubble to the midpoint between the two positions.Rotate by 90°
Next, rotate the instrument 90° in azimuth so that the plate level is now perpendicular to its previous position.Use the Third Screw for Adjustment
Use the third foot screw to move the bubble to its central position. Repeat the process as needed until the bubble remains centered for all instrument positions.Final Adjustment
Continue making adjustments until the bubble stays in the same central position regardless of how the instrument is rotated.
The final, centered position of the bubble is referred to as the “zero” of the level tube.
If the theodolite is equipped with two plate levels that are perpendicular to each other, the process is slightly simplified. In this case:
- Keep the longer plate level parallel to any two foot screws, and center the bubble by turning these screws simultaneously.
- Then, use the third foot screw to center the bubble of the second plate level.
Repeat the procedure until both bubbles remain centered in all positions of the instrument.
(b) Levelling with a Four-Screw Head
Levelling a theodolite equipped with a four-screw head follows a slightly different process compared to a three-screw model. The steps are as follows:
Position the Plate Level
Rotate the upper plate so that the longitudinal axis of the plate level is roughly parallel to two diagonally opposite foot screws, for example, screws D and B.Center the Bubble
Adjust both diagonally opposite foot screws (D and B) simultaneously in opposite directions to bring the bubble to the center of its run.Rotate by 90°
Turn the upper plate 90° so that the plate level is now parallel to the other two diagonally opposite foot screws, such as A and C.Center the Bubble Again
Adjust screws A and C to bring the bubble to the center of its run, just as in the previous step.Repeat for Precision
Continue repeating this process, adjusting the screws and checking the bubble’s position, until the bubble stays centered in all positions of the instrument.
(iii) Elimination of Parallax
Parallax refers to the apparent shift in the position of an object caused by a change in the observer’s eye position. In surveying with a theodolite, parallax occurs when the image formed by the objective lens is not in the same plane as the crosshairs, making accurate alignment difficult. To achieve precise bisection and sighting of objects, it is essential to eliminate parallax.
Parallax can be eliminated by properly focusing both the eyepiece and the objective lens, ensuring the image of the object aligns with the plane of the crosshairs. The process is detailed below:
Focusing the Eyepiece
To focus the eyepiece and achieve a clear view of the crosshairs:
- Hold a white paper in front of the objective lens or point the telescope towards a bright, featureless background like the sky.
- Move the eyepiece in or out until the crosshairs appear sharp and clearly defined.
Focusing the Objective
Once the crosshairs are clearly focused, the next step is to focus the objective lens:
- Direct the telescope towards a well-defined, distant object.
- Use the focusing knob to adjust the objective lens until the object appears sharp and crisp.
- To verify that parallax has been removed, move your eye slowly from side to side. If the object remains perfectly intersected by the crosshairs, the parallax has been successfully eliminated.
Checking for Parallax
If the object appears to shift when the observer’s eye moves:
Far Parallax: If the image of the object seems to move in the same direction as the eye, this indicates that the image and the observer’s eye are on opposite sides of the vertical crosshair. To correct far parallax, adjust the objective to bring the image closer to the observer’s eye.
Near Parallax: If the image moves in the opposite direction of the eye movement, this indicates that the image and the observer’s eye are on the same side of the crosshair. To remove near parallax, adjust the objective to increase the distance between the image and the observer’s eye.
By following these steps, parallax can be eliminated, ensuring accurate and precise measurements.
Permanent Adjustments of a Theodolite
Permanent adjustments of a transit theodolite ensure the instrument’s accuracy and proper functioning over time. These adjustments are typically made at the factory but may require occasional fine-tuning in the field. The main permanent adjustments include:
- Adjustment of the Horizontal Plate Level
- Adjustment of the Horizontal Axis (Trunnion Axis)
- Adjustment of the Telescope
- Adjustment of the Telescope Level
- Adjustment of the Vertical Circle Index
Adjustment of the Horizontal Plate Level
This ensures the plate levels are perpendicular to the vertical axis of the theodolite.
Test: Rotate the theodolite 180° after leveling with the foot screws. If the bubble stays centered, no adjustment is needed.
Adjustment: If off-center, adjust the bubble position using the foot screws and fine-tune with the capstan screws.Adjustment of the Horizontal Axis (Trunnion Axis)
Ensures the line of collimation moves in a vertical plane.
Test: Conduct a spire test by sighting a high point, then swinging the telescope 180°. If the sighting line doesn’t match the ground point, an adjustment is necessary.
Adjustment: Adjust the horizontal axis bearing screws to correct the alignment.Adjustment of the Telescope (Horizontal and Vertical Hairs)
Ensures accurate leveling and vertical angle measurements.
Test: Sight a distant point and reverse the telescope. If the readings differ, adjustment is required.
Adjustment: For the horizontal hair, adjust using the vertical diaphragm screws. For the vertical hair, use horizontal screws to align the sighting line.Adjustment of the Telescope Level
Ensures the line of collimation is horizontal when the bubble is centered.
Test: Measure height differences between two distant points, then reposition and repeat.
Adjustment: If the differences vary, adjust the level tube screws until the readings match.Adjustment of the Vertical Circle Index
Ensures accurate vertical angle readings.
Test: Observe a high and low point in the same plane. Reverse the telescope and recheck.
Adjustment: Use the vertical index adjustment screws to correct any errors in the angle readings.
